Electron gun for slalom focusing systems



y 1960 J. s. COOK ET AL 2,939,034

ELECTRON GUN FOR SLALOM FOCUSING SYSTEMS Filed June 10, 1955 J. s. coo/rINVEN TORS R. KOMPFNER m H. roco/w BY Ahab,

A 7' TORNE Y ELECTRON GUN FOR SLALOM FOCUSING SYSTEMS Filed June 10,1955, Ser. No. 514,421

8 Claims. (Cl. 315-35) This invention relates to an electron beamsystem, and more particularly relates to an electron gun for use inelectron'beam systems which employ electrostatic fields for focusing theelectronbeam. v

A broad object of the invention is to facilitate the injection of anelectron beam into an electron beam system of the kind in which theelectron beam must be injected with a particular velocity on anequipotential surface of an electrostatic field in a manner which littledisturbs the electrostatic field.

It will be convenient to describe the principles of the invention withspecific reference to an electron beam system for which it isparticularly well adapted, although the application of its generalprinciples may be extended beyond such limited use.

In particular, in a copending application of R. Kompf ner and W.H.,Yocom, Serial No. 514,423, filed June, 10, 1955, now United StatesPatent No.'2,857,548, issued Oct. 21, 1958, there is disclosed anelectron beam system in which a focusing electrode system is used toestablish an electrostatic field having a pair of singular equipotentialsurfaces,'which are'characterized by the fact that electrons injected oneither of such surfaces with a correct velocity will have a stabletrajectory on such surface. In an illustrative form of an electron beamsystem of this kind, a linear array of elements, each of which ismaintained at a positive potential with respect to the electron source,comprises the focusing electrode system which sets up a pair of singularequipotential surfaces. Each of these surfaces winds sinuously betweensuccessive elements of the array, the two surfaces being mirror imageswith the plane of the linear array as the'refiection plane. Inparticular, the correct velocity which the electron beam should havewhen injected on one such surface for stable flow therealong is achievedby associating with the electron source an accelerating anode which ismaintained at the potential of the-singular equipotential surface.

In the use of an electron beam system of this kind, it is foundrelatively difficult to inject by usual expedients an electron beam ofappreciable current onto one ofthe singular equipotential surfaces withthe correct velocity.

vIn particular, it is found that conventional beam injectionarrangements result, at the region of injection of the electrostaticfield, in a distortion of the field pattern, set up by the focusingelectrode system, optimum for focusing of this kind. This, in turn,results in a loss of focusing efiiciency. Accordingly, a specific objectof the present invention is to improve an electron beam system of thekind described by facilitating the injection of an electron beam withthe correct velocity on the singular equipotential surface which is toserve as the trajectory of the electron flow.

To this end, for isolating the electron source from the focusingelectrode system, a feature of the invention is an electrode having onesurface so shaped and positioned to coincide with the first of the twosingular equipotential United States Patent ,0

surfaces associated with the focusing electrode system and being slottedat the point where the second singular equipotential surface interceptsthe electrode surface. This electrode is maintained at the potential ofthe two singular equipotential surfaces, and an electron gun, typicallyof conventional design, is positioned behind the electrode. The beamprovided by the gun is injected on the second equipotential surface byway of the slot in the electrode.

The region of the slot is essentially a region of zero electrostaticfield, and, accordingly, there will be no forces acting on the beam atits point of injection on such singular equipotential surface. Thismakes possible smooth injection of the beam on this surface with thecorrect direction of velocity.

The invention will be better understood from the following more detaileddescription taken in conjunction with the accompanying drawing whichshows in a cross sectional view an electron gun in accordance with theinvention incorporated in an electron beam system of the kind describedin the aforementioned copending application.

In the electron beam system 10 depicted, a plurality of conductiveelements 11, which typically are wires extending transversely normal tothe plane of the drawing, are aligned to form a linear array whichextends longitudinally in the direction desired for flow. On oppositesides, conductive members 12 and 13 serve as conductive boundaries. Asdiscussed in the aforementioned application, 7

when the conductive elements 11 are maintained at a positive potentialwith respect to the conductive members 12, 13, (as is shownschematicallyin the drawing) this electrode system will establish in the interspacebetween the conductive members an electrostatic field which ischaracterized by a pair of singular equipotential surfaces whichintersect the plane of the drawing, as shown by the broken lines 14 and15. Each of such equipotential surfaces winds sinuously past successiveelements, the two surfaces being mirrorimages of one another about theplane of the linear array and intersecting one another along a parallelsuccession of center lines. Moreover, it is found that there will fiowadjacent such a surface an electron beam injected on either of suchsurfaces with a velocity in the direction of the surface and of amagnitude corresponding to the speed imparted to an electron in beingaccelerated from an electron source by an electric field of strengthcorresponding to the difference in potential of the singularequipotential surface and the electron source. The potential of thesingular equipotential surface can be adjusted to a desired value byproper choice of the potentials at which the elements of the focusingelectrode system are operated.

The electron beam is injected by an electron gun comprising a cathode16, a beam focusing electrode 17 and an accelerating anode 18. Since theelectron gun advantageously is to provide a strip beam which extendsnormal to the plane of the drawing, the various gun elements also extendnormal to the plane of the drawing. The cathode 16 is of conventionaldesign, including a heater compartment 16A in which extends a heatingcoil 16C and having a portion 16B of its surface which iselectron-emissive. The electrons emitted from the electronemissiveportion are formed into an electron beam by the beam forming electrode17. The accelerating anode 1S cooperates with electrode 17 incollimating the electrons into a beam and additionally serves toaccelerate the electrons in the beam to a desired velocity. Typically,the cathode is maintained at a potential slightly positive with respectto the beam forming electrode but considerably negative with respect tothe accelerating anode. The surface 18A which is more proximate to thecathode typically (although not necessarily) coincides with anequipotential surface associated with the electrode system of the gun.To the extent described up to this point, the electron gun isessentially of conventional design.

In accordance with the novel principles of the present invention, theportion of the surface 18B of the accelerating anode which is moreproximate to the focusing electrode system made up of the linear arrayof elements 11 and the conductive blocks 12, 13 is shaped to coincidesubstantially with surface 15 of the two singular equipotential surfacesassociated with such focusing electrode system. The length of suchportion is approximately between a quarter and a half of the period ofthe sinuous surface 15. The accelerating anode 18 is maintained at thepositive potential with respect to the cathode 16 which corresponds tothe potential of the singular equipotential surface 15. Additionally,the electron gun is arranged so that the electron beam is projectedthrough an aperture 19 provided in the accelerating anode 18 at a regionwhich corresponds to the interception of the anode surface 188 by thesingular equipotential surface 14. It is characteristic of thiscrossover region that no electric field is there present. Accordingly,since this is a region of approximately zero electric field, theelectron beam may here be injected smoothly on singular equipotentialsurface 15 for flow therealong.

It is also characteristic of the beam injection arrangement describedthat the presence of the electron gun will be effectively shielded fromthe focusing electrode system by the effect of the surface 18B of theaccelerating anode 18. Similarly, although of lesser importance, thepresence of the focusing electrode system will little affect theoperation of the electron gun because of the interposition of surface18A of the accelerating anode. Accordingly, undesirable interactionbetween the electron gun and the focusing electrode system can besubstantially eliminated.

At the downstream end of the linear array of elements, the electron beammay be collected by a target electrode 20 which is maintained at asuitably positive potential with respect to the electron source.Typically the target electrode is maintained at the potential of theelements 11 of the array. To minimize the effect of the target electrodeon the field pattern set up by the focusing electrode system, the targetelectrode is positioned behind an isolating electrode 21. This electrode21 is slotted at the region of interception by the singularequipotential surface 14 on which the electron beam flows and thesurface 21A of this electrode is made to coincide with a portion of theother singular equipotential surface 15 and maintained at the D.-C.potential characteristic of this equipotential surface. electrode 20 iseffectively shielded from the electrostatic field established by thefocusing electrode system.

In a typical arrangement, the wire elements 11 were of 60 mils diameterwith a center to center spacing of 150 mils, and were maintained at apotential of +350 volts. The conductive plates 12 and 13 were spacedapart 180 mils and maintained at a potential of l90 volts. Theaccelerating anode 18 was maintained at a voltage of +225 volts. Thecathode 16 was at ground potential and the beam focusing electrode 17about one half a volt positive.

It is to be understood that the specific arrangement described is merelyillustrative of the general principles of the invention. Various otherembodiments may be devised without departing from the spirit and scopeof the invention. In particular, various modifications may be made inthe focusing electrode system used to establish the pair of singularequipotential surfaces. For example, either or both of the conductivemembers 12, 13 may be eliminated or may comprise a linear array of wireelements, so long as the necessary compensation is made in the otherparameters of the focusing electrode system as described more fully inthe above-mentioned Kompfner- Yocorn copending application.Alternatively, the focusing electrode system may be of the kinddescribed in copend- As a consequence, the presence of the target 4 ingapplication Serial No. 514,422, filed June 10, 1955, by C. F. Quate andJ. W. Sullivan, now United States Patent 2,849,650, issued August 26,1958,

What is claimed is:

1. In an electron beam system, means defining a region for traversal byan electron beam comprising an electron beam source and a collectorelectrode, a focusing electrode system which establishes in said regiona pair of singular equipotential surfaces which intersect at at leastone point in said region, and an arrangement for injecting said beaminto said region on one of said surfaces for flow therealong comprisingsaid electron beam source and an accelerating anode characterized inthat a portion of the surface of the accelerating anode substantiallycoincides with one of said singular equipotential surfaces and saidanode surface is apertured for passage of the electrons therepastsubstantially at the region of its intersection with the other of saidsingular equipotential surfaces.

2. In an electron beam system, means defining a region for traversal byan electron beam comprising an electron source and a collectorelectrode, a focusing electrode system which establishes in said regiona pair of singular equipotential surfaces which intersect at at leastone point in said region, means including said electron source forproviding an electron beam for injection into said region on one of saidsurfaces for flow therealong, and means for isolating the electronsource from the focusing electrode system comprising a conductiveelement having a. portion of its surface substantially coincident withone of said equipotential surfaces said surface being apertured forpassage of the electron beam therepast substantially at the region ofintersection with the other of said singular equipotential sur faces.

3. In an electron beam system, means defining a region for traversal byan electron beam comprising an electron gun and a collector electrode, afocusing electrode system which establishes in said region a pair ofsingular equipotential surfaces which intersect at at least one point insaid region, said electron gun being positioned to direct an electronbeam for flow along one of said singular equipotential surfaces, andmeans for isolating the electron gun from the focusing electrode systemcomprising a conductive portion which is maintained at the potential ofsaid singular equipotential surfaces and positioned intermediate betweenthe gun and the focusing electrode system having a surface coincidentwith one of said equipotential surfaces over an extended region, saidsurface being apertured for passage of the electron beam therethroughsubstantially at the region of intersection with the other of saidsingular equipotential surfaces.

4. In an electron beam system, a focusing electrode system whichestablishes a pair of singular equipotential surfaces characterized byat least one intersection with each other, an electron source and acollector positioned at opposite ends of the focusing electrode systemfor projecting a electron beam therepast for flow along one of saidsingular equipotential surfaces, and means adjacent separate ends of thebeam path for isolating the electrode beam system from the electronsource and the collector, each of said means comprising a conductiveelement having a portion of its surface substantially coincident withone of the singular equipotential surfaces and apertured for passage ofthe electron beam therethrough at the region of intersection with theother of said singular equipotential surfaces.

5. An electron discharge device comprising spaced inner and outerconducting members defining a region for traversal by an electron beam,means for establishing a pair of equipotential surfaces in the spacebetween said members which intersect at at least one point in saidregion, and electron gun means adjacent one end of said region forprojecting an electron beam substantially onto one of said surfaces forflow therealong, said gun means comprising an electron source and anaccelerating anode having an outer surface portion substantiallycoinciding with one of said equipotential surfaces and being aperturedfor passage of electrons therethrough substantially at the point ofintersection of said one surface with the other of said equipotentialsurfaces.

6. An electron discharge device in accordance with claim furthercomprising a collector electrode at an end of said region, and aconductive element positioned between said collector electrode and saidregion, said conductive element having an outer surface substantiallycoinciding with said one equipotential surface and being apertured forpassage of electrons therethrough substantially at the region of theintersection of said one surface with said other surface.

7. An electron discharge device comprising a plurality of linearconducting elements positioned in an array, means for establishing apair of singular equipotential surfaces encompassing each of saidelements and intersecting between adjacent of said elements, andelectron gun means adjacent one end of said array for projecting anelectron beam substantially onto one of said surfaces for flowtherealong, said electron gun means comprising an electron source and anaccelerating anode having an outer surface portion substantiallycoinciding with one of said singular equipotential surfaces and beingapertured for passage of electrons therethrough substantially at theregion of the intersection of said one surface with the other of saidequipotential surfaces.

8. An electron discharge device in accordance with claim 7 furthercomprising a collector electrode at the other end of said arraypositioned on said other equipotential surface, and a conductive elementpositioned between said collector electrode and the last of said linearelements in said array, said conductive element having an outer surfacesubstantially coinciding with said one singular equipotential surfaceand being apertured for passage of electrons therethrough substantiallyat the region of the intersection of said one surface with said otherequipotential surface.

References Cited in the file of this patent UNITED STATES PATENTS2,296,355 Levin Sept. 22, 1942 2,638,561 Sziklai May 12, 1953 2,687,491Lee Aug. 24, 1954 2,694,783 Charles Nov. 16, 1954 2,702,370 Lerbs Feb.15, 1955 2,790,106 Labin Apr. 23, 1957 2,807,739 Berterottiere et al.Sept. 24, 1957 FOREIGN PATENTS 853,009 Germany Oct. 20, 1952

